Dark current is charge generated in an image sensor when the image sensor is not exposed to light. Dark current is detrimental to device performance because the dark current decreases the sensitivity of the image sensor. And reduced sensitivity in an image sensor degrades the quality of image captured in low light conditions.
FIG. 1 is a cross-sectional view of a portion of a first image sensor according to the prior art. Image sensor 100 includes substrate 102 and epitaxial layer 104. Photosensitive regions 106 formed in epitaxial layer 104 are used to capture images by collecting photo-generated charge in response to incident light. Dark current “c” generated in image sensor 100 can diffuse to photosensitive regions 106 and combine with the photo-generated charge during image capture. Dark current adversely affects the quality of the captured image because the amount of charge collected by the photosensitive regions is incorrect and does not accurately reflect the scene being captured.
There are several sources for dark current in image sensor 100. One source is the interface state defects 108 that reside at the interface 110 between substrate 102 and epitaxial layer 104. Another source is metal contaminates, such as iron and gold, in both substrate 102 and epitaxial layer 104.
During the fabrication process of an image sensor, oxygen in substrate 102 forms oxygen precipitates 112. The oxygen precipitates 112 getter metal contaminates. Higher concentrations of oxygen lead to more oxygen precipitates and better gettering properties. When the oxygen precipitates 112 are close to interface 110, dark current generated by the metal atoms trapped at the precipitate sites and by the defects associated with the precipitates, can diffuse to photosensitive regions 106. One solution to this issue is to form the oxygen precipitates 112 a given distance away from interface 110.
FIG. 2 is a cross-sectional view of a portion of a second image sensor according to the prior art. Image sensor 200 includes region 202 that is depleted of oxygen so that oxygen precipitates 112 do not form in this region. Region 202 is part of the substrate 102 and is also known as a denuded zone. The thickness of region 202 depends on the oxygen concentration inside substrate 102 and the type of processes used to create that concentration. In general, for a given process, the thickness of region 202 increases as the oxygen concentration in substrate 102 decreases. However, as discussed earlier, higher oxygen concentration is preferred in order to create more oxygen precipitates and achieve better gettering performance. Additionally, creating region 202 usually requires thermal processing of substrate 102, before epitaxial layer 104 deposition, and at high temperatures for a long period of time. In a production environment that involves thousands of wafers, these extra processing steps can limit throughput and increase costs. And since these additional processing steps involve high temperatures, metal contaminants can easily diffuse in substrate 102 during these steps.